backends/x11: Drop unused X11WindowedBackend::sizeChanged signal

src/backends/x11/windowed/x11_windowed_backend.cpp

@@ -555,7 +555,6 @@ void X11WindowedBackend::updateSize(xcb_configure_notify_event_t *event)
     if (s != output->pixelSize()) {
         output->resize(s);
     }
-    Q_EMIT sizeChanged();
 }
 
 xcb_window_t X11WindowedBackend::rootWindow() const

src/backends/x11/windowed/x11_windowed_backend.h

@@ -137,9 +137,6 @@ public:
 
     Outputs outputs() const override;
 
-Q_SIGNALS:
-    void sizeChanged();
-
 private:
     void createOutputs();
     void grabKeyboard(xcb_timestamp_t time);